1. Field of the Invention
The present invention pertains to LADAR systems, and, more particularly, to a LADAR transmitter for use in a scanned illumination implementation.
2. Description of the Related Art
Many military and civilian remote sensing applications rely on optical techniques such as laser detection and ranging (“LADAR”). At a very high level, LADAR works much like the more familiar RADAR (“radio wave detection and ranging”), in which radio waves are transmitted into the environment and reflected back, the reflections yielding range and position information for the objects that generate them. LADAR does roughly the same thing, but using light rather than radio waves. Although there are some significant differences in performance, they are similar in at least this one basic respect.
One type of LADAR system employs what is known as a “scanned illumination” technique for acquiring data. More technically, a LADAR transceiver aboard a platform transmits the laser signal to scan a geographical area called a “scan pattern”. The laser signal is typically a pulsed, split-beam laser signal. That is, the laser signal is typically transmitted in short bursts rather than continuously. The LADAR transceiver produces a pulsed (i.e., non-continuous) single beam that is then split into several beamlets spaced apart from one another by a predetermined amount. Each pulse of the single beam is split, and so the laser signal transmitted in is actually, in the illustrated embodiment, a series of grouped beamlets. The LADAR transceiver aboard the platform transmits the laser signal. The laser signal is continuously reflected back to the platform, which receives the reflected laser signal. Note, however, that some implementations employ a continuous beam, an unsplit beam, or a continuous, unsplit beam.
Each scan pattern is generated by scanning elevationally, or vertically, several times while scanning azimuthally, or horizontally, once within the field of view for the platform. Thus, each scan pattern is defined by a plurality of elevational and azimuthal scans. The principal difference between the successive scan patterns is the location of the platform at the start of the scanning process. An overlap between the scan patterns is determined by the velocity of the platform. The velocity, depression angle of the sensor with respect to the horizon, and total azimuth scan angle of the LADAR platform determine the scan pattern on the ground. Note that, if the platform is relatively stationary, the overlap may be complete, or nearly complete.
The platform typically maintains a steady heading while the laser signal is transmitted at varying angles relative to the platform's heading to achieve the scans. The optics package of the LADAR transceiver that generates and receives the laser signal is typically “gimbaled”, or mounted in structure that rotates relative to the rest of the platform. Exemplary gimbaled LADAR transceivers are disclosed in:                U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,” issued Apr. 6, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky, et al.; and        U.S. Pat. No 5,224,109, entitled “Laser Radar Transceiver,” issued Jun. 29, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky, et al.However, there are many alternatives known to the art.        
For technical reasons, the entire optics package is typically gimbaled. More particularly, in conventional systems, the components that comprise the optical train through which the laser signal is generated and transmitted must be optically aligned. This optical alignment cannot be achieved when a part of the optical train is moving relative to the rest of the optical train. Thus, the LADAR transceiver has “on-gimbal” laser cavities and bulk optics to expand, collimate, segment, and align the laser output. This adds size, weight, complexity, and cost to the LADAR transceiver. The on-gimbal laser cavity also requires a fiber coupled Laser diode pump which is a significant cost driver. Furthermore, current delivery and alignment techniques for the bulk optics are inefficient, sensitive to tolerances and temperature, and limit the output power per channel and therefore limits the signal-to-noise ratio in a multi-beam LADAR system.
The art has not found a successful solution to these types of problems associated with conventional gimbaled LADAR transmitters/transceivers. One approach employs a fiber laser to mitigate some of these problems. However, current fiber lasers and mode coupled fiber delivery approaches are limited either in their power tolerance (i.e., laser induced 5 damage threshold, or “LIDT”) or laser beam quality (e.g., times diffraction limit, or M2) because they tend to rely on a single fiber optic channel. For example, a conventional single mode optical fiber has a very small mode field diameter, and therefore, higher energy densities at its fiber/air interface and lower LIDT. Increasing the mode field diameter without limiting the number of guided modes may improve LIDT, but it increases output M2 reducing delivered beam quality.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.